To be able to 'cash' them in and receive steem, would require a hardfork.
Without that, the idea is burning them would peg them to STEEM 1:1, although there's nothing in place to enforce the peg. The only way to cash them in would be to trade them with someone for STEEM or something else.
I'm hoping perhaps it can be automated with an SMT market maker and oracle contract, that would provide a way to go from black pocket back to STEEM and vice versa. ( Like I said though, no idea if this is possible )